Insulation Calculator: Bags and Rolls of Batt
Gross area less the framing, divided by what a bag actually covers. The framing factor is the part that gets skipped, and it is 7 to 10 percent of every wall.
When the figures look right, the insulation material list downloads as a real PDF with your business details on it, built inside your own browser. Nothing you type is uploaded anywhere.
Worked example
These are the figures the calculator opens with and the answer it gives. Change anything above and every number below moves with it.
- Wall or ceiling area900 sq ft
- Stud or joist spacing16 in
- Coverage per bag or roll106.8 sq ft
Bags or rolls8 bags
| Gross area | 900 sq ft |
|---|---|
| Framing factor at 16 in oc | 90% |
| Cavity area to insulate | 810 sq ft |
| Bags at 106.8 sq ft | 8 |
| Coverage bought | 854 sq ft |
The material list this produces
| Insulation batts, bags | 8 |
|---|---|
| Vapor retarder, sq ft (if required) | 900 |
What this calculator assumes
- Framing takes up roughly 10 percent of a wall at 16 inch on center and 7 percent at 24. Cavity area is reduced accordingly, so you do not buy insulation for the studs.
- Bag coverage varies a lot by R value and width. The default is a common R-13 kraft faced batt bag; read your own bag and change it.
You do not insulate the studs
A wall framed at 16 inches on center is roughly 10 percent lumber once you count the plates, the studs, the headers and the corners. At 24 inch centers it drops to about 7 percent. That is cavity you are not filling, so it is material you should not be buying.
The same framing is also why a wall of R-13 batts does not perform at R-13. Wood conducts heat far better than fiberglass, so every stud is a small thermal bridge. Continuous exterior insulation is the answer to that, and it is a separate calculation.
Read your own bag
Bag coverage moves with R value, width and thickness, from around 40 square feet for a thick R-30 bag to over 130 for a thin R-11. The default here is a common R-13 kraft faced batt, and it is a default, not a fact about your order.
Match the width to the framing. A 15 inch batt is for 16 inch centers and a 23 inch batt is for 24. Squeezing a wide batt into a narrow bay compresses it and loses R value, and cutting a narrow one to fit wide bays leaves gaps at the edges, which loses more.
Questions people ask
How much insulation do I need for a 1,000 square foot wall?
About 900 square feet of cavity at 16 inch centers, once the framing is deducted. At roughly 106.8 square feet a bag that is 9 bags.
Free Contractor Estimate Template, free, no signup, PDF Word and Excel.
Common sizes, worked out
The same calculator with the numbers already filled in, one page per size, each ending in its own material list.
More sizes, answered
Each row is this calculator run with those numbers when the page was built. Type the same numbers in above to see every line of the working.
| Example | What it assumes | Bags or rolls |
|---|---|---|
| 300 sq ft | 300 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 3 bags |
| 10x10 room | The four walls of a 10 by 10 room at 8 foot ceiling height, 40 linear feet of wall, framed at 16 inches on center. | 3 bags |
| 10x12 room | The four walls of a 10 by 12 room at 8 foot ceiling height, 44 linear feet of wall, framed at 16 inches on center. | 3 bags |
| 12x12 room | The four walls of a 12 by 12 room at 8 foot ceiling height, 48 linear feet of wall, framed at 16 inches on center. | 4 bags |
| 400 sq ft | 400 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 4 bags |
| 12x16 room | The four walls of a 12 by 16 room at 8 foot ceiling height, 56 linear feet of wall, framed at 16 inches on center. | 4 bags |
| 500 sq ft | 500 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 5 bags |
| 16x20 room | The four walls of a 16 by 20 room at 8 foot ceiling height, 72 linear feet of wall, framed at 16 inches on center. | 5 bags |
| 600 sq ft | 600 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 6 bags |
| 20x20 garage | The four walls of a 20 by 20 garage at 8 foot plate height, 80 linear feet of wall, framed at 16 inches on center. | 6 bags |
| 700 sq ft | 700 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 6 bags |
| 20x24 garage | The four walls of a 20 by 24 garage at 8 foot plate height, 88 linear feet of wall, framed at 16 inches on center. | 6 bags |
| 900 sq ft | 900 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 8 bags |
| 1000 sq ft | 1,000 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 9 bags |
| 1,100 sq ft | 1,100 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 10 bags |
| 1,300 sq ft | 1,300 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 11 bags |
| 1500 sq ft | 1,500 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 13 bags |
| 1,500 sq ft attic | A 1,500 square foot attic floor, joists at 24 inches on center, with 106.8 square foot bags. | 14 bags |
| 1,600 sq ft | 1,600 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 14 bags |
| 1,800 sq ft | 1,800 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 16 bags |
| 2000 sq ft | 2,000 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 17 bags |
| 2,500 sq ft | 2,500 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 22 bags |
| 3,000 sq ft | 3,000 square feet of wall or ceiling, framed at 16 inches on center, with 106.8 square foot bags. | 26 bags |